I don't know what state you are in, but in some states the state vet will do necropsies on the bodies to find out the cause of death for a small amount of money, and in California, for free. You really need to send or take the next chicken that dies in for a necropsy. Their is definitely something going on. Lice, mites, coccidiosis, or some other disease is likely killing them. Have you noticed any other symtoms? I would probably give a 5 day course of Corid liquid (2 tsp per gallon of water,) as a treatment for cocci, just to see if the deaths stopped.
